Way, way, long time ago there was a small group of hoofed predatory mammals. They didn' t look nuthin' like deer (at least their skeletons don' t look like deer skeletons).
There are many animals that don' t have teeth now, but did when they were in their mom' s womb. Many developing birds have tooth buds in theri beaks, but the buds are absorbed before hatching. The deer teeth you' re talking about are probably much the same.